Many attempts have been made to electrically simulate the acoustic environment of large halls and auditoriums. It is common for such devices to utilize a plurality of means for delaying the original electrical signal, each delay means delaying the original electrical signal a different amount of time. In order to increase the echo density of the reverberated signal using the minimum number of delay means, such apparatus frequently utilize means for reentering the delayed signals into the time delay means. The reentered signals, referred to as "reflections", are reentered at different points in the delay means so that the additional delay time between the originally delayed signal and the first reflection will vary from significantly less than the original delay time to as much as twice the original delay time.
Typical of such reverberation apparatus are apparatus which use helical spring delay means. Several such springs are connected at one end to a source of electrical signals which vibrate the springs. The opposite ends of the springs are connected to some means for converting from the mechanical vibrations back to the corresponding electrical signals at an output terminal or terminals. The delayed signals are then recombined with the original source signals at some point prior to converting the electrical signals into the sounds which they represent. The springs are of different total delay times. Additionally, because of the mechanical reflections at the output of each spring, the springs are equipped with a natural form for reentering the signal into the delay means. The spring vibration which is partially reflected off the output end of the spring will then travel down the spring to be partially reflected off the input end of the spring. As a result, a reflection will be received at the output end of each spring after three times the spring delay time of the original signal which caused the reflection. This represents an additional delay of the signal of twice the delay time of the spring following the receipt at the output of the original delayed signal. Additional reflections can be obtained from discontinuities in the spring such as shown in numerous patents which will be referred to.
Reverberation apparatus are also known which use electronic time delay means. Typical of such time delay means is the common charge coupled or "bucket brigade" type. The output signal can then be fed back into the bucket brigade at any point to create a reflection. If fed back at the beginning of the time delay means, then the additional time between the original delay signal and the reflection is equal to the original delay time. The total delay from the time the original signal is received by the delay means to the receipt of the reflection at the output is twice the delay time for the original delayed output. A second reflection will then be received at three times the original delay time, and so on.
Time delay means have also been constructed which make use of tapped output signals. In such a system, the delay time from the input to the first tap operates as a first time delay means, the delay time from the input to the second tap operates as a second time delay means, and so forth.
The methods for making the apparatus having the time delay means have varied considerably. The time delay means have been chosen so that the intervals between the natural frequencies of the device are constant as shown in U.S. Pat. No. 2,923,369 (Kuhl). Others have merely used appreciably different spring lengths as mentioned in U.S. Pat. No. 2,982,819 (Meinema) without regard to the actual differences between the delay times. Some delay times have simply been found to be satisfactory. For instance, a two spring systems with delay times of 37 milliseconds in one spring and 29 milliseconds in the other are mentioned as satisfactory in U.S. Pat. No. 3,106,610 (Young) and in U.S. Pat. No. 3,159,713 (Laube).
In other systems, spring lengths have been chosen to be resonant at certain frequencies. U.S. Pat. No. 3,281,724 (Schafft) chose the use of springs which are resonant at very low frequencies for efficient energy transfer at all of the harmonics of the resonant frequencies. Two springs of identical characteristics are used, one spring being fixed at both ends to be resonant at one-half wave length and the other spring being fixed at one end and freely suspended at the other to be one-quarter wave length resonant. U.S. Pat. No. 3,431,516 (Schafft) shows the use of coupling links between the two springs at unspecified locations. U.S. Pat. No. 3,391,250 (Klaiber) shows the use of a single spring device with damping in an attempt to overcome the reinforcement and damping problems encountered with helical springs having different natural periods of vibration.
U.S. Pat. No. 3,363,202 (Meinema) and U.S. Pat. No. 3,347,337 (Mochida et al.) show multiple spring devices which use springs of unspecified different lengths and characteristics. U.S. Pat. No. 3,564,106 (Pavia) mentions only that the two springs should have slightly different transmission characteristics, whereas U.S. Pat. No. 3,402,371 (Weingartner) which uses coil springs having different diameters arranged concentrically mentions only that the ratio of delay times associated with the springs is preferably an irrational number.